This invention relates to gyroscopes in general and more particularly to an improved vibratory gyroscope.
Vibratory gyroscopes may be rate integrating vibratory single axis gyroscopes used to facilitate navigation tasks specifically in regard to strap down inertial navigation of vehicles, such as airplanes. Vibratory gyroscopes may also be employed as rate gyroscopes used primarily as sensors for stabilization purposes. In the past, it has been customary, however, to use rotary gyroscopes to obtain inertial rotation reference to one or more axis in a moving vehicle. The main problem with such gyroscopes is that they employ bearings that have limited life. The life expectancy is typically only five thousand hours. To overcome this problem hydrodynamic bearings have been developed. Their main drawbacks are excessively small bearing gaps, high running power and limited capability to be stopped and started due to wear, and extremely high cost.
In search for better gyroscopes, the development of linear vibrating angular direction sensors has taken place. One such sensor is disclosed in U.S. application Ser. No. 498,035 filed Aug. 16, 1974 and assigned to the same assignee as the present invention. As fully disclosed therein the vibrating gyroscope utilizes conservation of linear momentum to sense angular rotation about its input axis. Although the previously disclosed sensor offers many advantages is does have a number of drawbacks. The previously disclosed vibrating gyroscope utilizes two necked down shafts to suspend the vibrating elements from the casing. The flexures must be made relatively weak in bending compared with the rod joining the two vibrating inertial assemblies in order to insure a high Q value for the sensor and consequently a low drift. The rod is preferably made of a material with a high Q value as ceramic. The flexures must be made of a tough, high-strength material such as a titanium alloy Ti-6% Aluminum-4% Vanadium. This is necessary so that the instrument can withstand rough handling and a severe vibrational environment such as that found in an aircraft where such sensors may be used in a strapped down mode. However, materials of this strength and toughness also exhibit low Q values. That is to say they have high internal energy losses. Fortunately, in the previously disclosed instrument the requirement for high Q value on one hand and a reasonable shock susceptibility on the other hand can be compromised. However, in reaching this compromise a third important characteristic is impaired: the suspension is weak in the radial and torsional directions. As a result, a relatively low resonant frequency exists in the radial direction. In the prior art instrument this frequency is approximately 400 Hz. As a result, this frequency is within the pass band specified in MIL-E-5400 which may be used in evaluating instruments of this nature. Furthermore, the low frequency results in an increased coupling between the instrument and the environment resulting in a larger drift due to unbalances than is desired. Thus, an increase of the resonant frequency by approximately four times to a frequency of 1600 Hz in the radial direction would be desirable. This would make the resonant frequency higher than the test pass band of MIL-E-5400. The coupling between the vibrating parts in the instrument and the environment would also be considerably reduced. This will result in a large reduction of the drift due to the unbalances in the instrument. However, to modify the prior art instrument to obtain the four-fold increase in resonant frequency the stiffness of the flexures would have to be increased by a factor 4.sup.2. Unfortunately, to do so would seriously impair the design tradeoff mentioned above. That is to say the flexures would then, instead of sharing two percent of the total potential energy with the flexible rod, assume 32 percent. It would then become impossible to achieve the high Q value needed for low drift operation.
An even more cumbersome problem in the prior art instrument is the fact that the resonant frequency of the vibrating elements in torsion is quite low being in order of 30 Hz. It would be desirable to raise this frequency to above 1000 Hz. Obviously, it would be impossible to scale the dimensions of the prior art instrument to achieve such a result.
In view of this, the need for an improved instrument of this nature which does not suffer from these deficiencies and is of a simple construction becomes evident.